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The 20 potentially 'most Earth-like' worlds were chosen because the odds appear stacked in their favour.

"It's exciting to see the sheer amount of planets that are out there, which makes you think that there is zero chance of there not being another place where life could be found," says Michelle Hill, an undergraduate Australian student studying at San Francisco State University and co-author of the report.

But the list of 20 were chosen for further examination because they're not too close to their star for a 'runaway greenhouse effect' such as seen on Venus. Nor are they too far out - such as Mars - where their water would freeze.

"We can focus in on the planets in this paper and perform follow-up studies to learn more about them, including if they are indeed habitable," says the study's lead author, Stephen Kane, an associate professor of physics and astronomy at SF State.

"There are a lot of planetary candidates out there, and there is a limited amount of telescope time in which we can study them. This study is a really big milestone toward answering the key questions of how common is life in the universe and how common are planets like the Earth."